The present invention is related to mass fluid (gas or liquid) flow sensors which provide an output signal related to sensed fluid flow rate. More specifically, the present invention is related to fluid flow sensors which produce a signal related to sensed fluid flow in accordance with sensed heat transfer between a heater element positioned in the fluid flow to be sensed and the fluid.
Essentially, there are two types of fluid flow sensors which utilize a heater element and depend upon the heat transfer between the heater and the fluid to produce an output signal related to fluid flow. One of these types of fluid flow sensors comprises a constant power-type flow sensor in which constant power or constant current is supplied to a heater resistor. The heater resistor is positioned in the fluid flow to be measured. An adjacent temperature variable resistor is positioned downstream in the fluid flow and is utilized to sense the heat transfer between the heater and the fluid, and, with other circuitry, to produce a voltage related to fluid flow. Another type of sensor consists of a hot anemometer sensor in which a bridge circuit, which includes a heater resistor having a temperature variable resistance characteristic, is utilized to produce an output bridge balance voltage that determines a control signal which controls the amount of power dissipation of the heater resistor so as to provide the heater resistor at a predetermined temperature above fluid temperature. The heater resistor temperature for such "constant temperature" sensors is maintained constant for any fluid flow rate in the range of flow rates to be measured, assuming a contant temperature for the incoming fluid passing by the heater resistor. Each of these sensors use a sense circuit, thermally or electrically coupled to the heater, to develop a fluid flow rate signal. Both of these types of fluid flow sensors are well known, and each has been suggested for use in automotive vehicle applications for use in sensing the mass air flow into the vehicle engine or the fuel flow into the engine. The sensed air flow or fuel flow is then utilized by engine control electronics to optimize engine performance and fuel combustion.
In mass fluid flow sensors of the above-described types, typically it is difficult to cost effectively minimize flow error readings which occur due to changes in fluid temperature so that error flow readings are substantially independent of fluid temperature. Changes in fluid temperature can result in either changes in the heat transfer characteristics of the fluid or changes in the relative proportion of the types of heat transfer which exists between the heater and the fluid, such as changing the relative proportion of heat transfer which occurs due to conduction, free convection and/or radiation.
While some fluid flow sensors attempt to compensate for changes in fluid temperature by sensing fluid temperature and producing a compensation signal, typically the output signal of such sensors is not substantially independent of fluid temperature, thus resulting in errors in sensed fluid flow. While one prior constant power sensor has proposed developing a compensating signal related to sensed fluid temperature which has a variation that directly opposes the flow sense signal variation produced by a fluid sensor as a function of fluid temperature, this involves generating a substantially nonlinear temperature varying signal which must directly match the nonlinear variation of the sense signal as a function of temperature. Such compensation is difficult to achieve in a cost effective manner. Also, prior flow sensors have not recognized that heat transfer to the fluid is a complex function of both fluid temperature and flow rate so that providing the proper temperature compensation for one flow rate does not insure proper temperature compensation for a substantial range of flow rates.
The above problems exist even though most of the two different types of fluid flow sensors discussed above already utilize a temperature sensitive element for sensing fluid temperature. While this aids in improving sensing accuracy, additional accuracy is needed if such sensors are to be used for efficient vehicle engine control. Thus the prior art has not provided a cost effective technique for providing a sensed fluid flow rate signal which is substantially independent of fluid temperature and has improved accuracy.